Allergic asthma is a complex disease involving a localized inflammatory response mediated by lymphocytes, eosinophils and mast cells. One critical component of this response is the T helper type 2 (Th2) cytokines, including IL-4, IL-5, IL-9, and IL-13, produced by T cells and mast cells. The Tec family tyrosine kinases, Itk and Rlk, play important roles in activating phospholipase C-y1 downstream of the T cell receptor. Itk/Rlk- deficient T cells show impaired TCR signaling, leading to substantially reduced cytokine production and defects in clearing pathogenic infections. Interestingly, unimmunized Itk-/- and Itk-/-Rlk-/- mice exhibit abnormalities suggestive of excess Th2-type cytokine production, such as germinal center hyperplasia in the spleen, eosinophilia, and elevated levels of serum IgE antibodies. These data, together with the fact that Itk and Rlk are also expressed in mast cells, suggest the following hypothesis: Itk and/or Rlk may be critical in T cells for IL-4 production leading to Th2 cell differentiation and cytokine production, but may also play a role in mast cell cytokine production. To test this hypothesis, we propose to determine whether the abnormalities seen in unimmunized Itk-/- and Itk-/-Rlk-/- mice are due to a T cell-intrinsic defect, a defect in mast cells, or both. Second, we find that Itk-/- bone marrow-derived mast cells secrete enhanced levels of IL-4, IL-6, and IL-13 following FceRI stimulation. Based on these data, we hypothesize that Itk is required for optimal activation of the SHIP1/Dok-1 pathway, known to attenuate mast cell responses. To test this idea, we will examine Itk-/- mast cells for alterations in degranulation and cytokine production in response to stimulation with IgE alone, IgE plus antigen, SCF, as well as to FceRI and FcgRII co-aggregation. Biochemical analyses will directly address the role of Itk in the SHIP1-dependent pathway. The overall goal of these studies is to further our understanding of signaling pathways that may contribute to the development of allergic diseases, and to suggest mechanisms for the manipulation and/or prevention of allergic responses.